Battery charge monitor

Abstract

An arrangement for detecting the state of charge of a vehicle battery (B201) in an electrical system (Z201) of a vehicle comprises an array of diodes (BD201) arranged to detect the initial voltage drop across the load when energised by a switch (M201). Each diode in the array defines a predetermined forward voltage drop and the diodes are arranged to energise respective thyristors (S201 to S208) when they conduct. These thyristors energise respective LEDs (201 to 208) which form a bar graph display, whereby the greater the voltage drop across the load (Z201) the greater the number of diodes which conduct and the greater the number of LEDs which are illuminated. Accordingly the bar graph display indicates the state of charge of the battery.

I claim: 
     
       1. An arrangement for detecting and indicating the state of charge of a rechargeable battery, the arrangement comprising a discharge circuit arranged to be connected to terminals of the battery, the discharge circuit including a switch in series with a load, said load further comprising an electrical system of a motor vehicle including a starter motor of said vehicle, and means for detecting an initial voltage drop across said load, said voltage drop being dependent upon the internal resistance of the battery, and the arrangement further comprising a display device which is responsive to said voltage drop and arranged to display the state of charge of the battery. 
     
     
       2. An arrangement as claimed in claim 1 including means in said discharge circuit for intermittently controlling said switch to remove current through said load. 
     
     
       3. An arrangement as claimed in claim 1 wherein said detecting means comprises an array of series-connected diodes, the diodes being similarly poled and each defining a forward voltage drop, the junctions of the diodes being connected to respective bypass resistors whereby the number of diodes in the array which conduct in response to the application of a voltage drop across said array depends on the magnitude of said voltage drop across said array, the display being responsive to the number of diodes which conduct. 
     
     
       4. An arrangement as claimed in claim 3 wherein said display comprises a plurality of light-emitting diodes (LEDs) which are connected in parallel and arranged to be switched on individually in response to current passing through respective ones of said bypass resistors. 
     
     
       5. An arrangement as claimed in claim 4 wherein said light-emitting diodes are arranged to be switched on by respective gate-controlled semiconductor switching devices whose gate electrodes are connected to respective ones of said bypass resistors. 
     
     
       6. An arrangement as claimed in claim 1, further comprising a storage capacitor connected to the rechargeable battery for charging thereby, said storage capacitor charging to a voltage which is substantially unaffected by discharge of the battery through said discharge circuit, the detecting means being energized by said substantially unaffected voltage.